1. Field of the Invention
The present invention relates to a ceramic substrate.
2. Description of the Related Art
For manufacturing semiconductor devices, a heating/cooling apparatus to heat or cool a semiconductor wafer (hereinafter referred to as a “wafer”) is used. In the heating/cooling process, in order to set the wafer at a predetermined temperature distribution, temperature control conditions in the heating/cooling apparatus need to be specifically determined.
For determination of the temperature control conditions in the heating/cooling apparatus, a circuit substrate for holding sensors has been used. For example, a circuit substrate with sensors are is placed in a heating/cooling apparatus, and a plurality of sensors on the circuit substrate measure temperature fluctuation in the heating/cooling process. Based on the measurement result, the temperature conditions in the heating/cooling apparatus are adjusted so that a wafer can have desired temperature distribution during heating or cooling.
As an example of a substrate for holding sensors, a ceramic substrate is shown in Japanese Unexamined Patent Application Publication No. 8-17888 (Patent Document 1). The Patent Document 1 describes a substrate composed of a sintered polycrystalline aluminum oxide body having an average strength of 450 MPa or more, an average crystal grain size of 5 μm or less, a bulk density of 3,980 kg/m3 or more, and a purity of 99.9% by mass or more.
In recent years, the thickness of wafers has been reduced and the size of wafers has been increased. The thickness of circuit substrates has been reduced and the size of circuit substrates has been increased. As the thickness of circuit substrates becomes smaller and the size of circuit substrates becomes larger, the circuit substrate become likely to deflect or vibrate. For example, there may be cases where circuit substrates are deflected due to an acceleration applied to the sensor-holding substrates carried in and out of the heating/cooling apparatus, and then the circuit substrates vibrate. When temperature or the like is measured in a state where a circuit substrate vibrates, there may be a case where the temperature cannot be measured accurately because malfunctions of sensors, errors in sending or receiving signals to or from sensors, or the like occur. Therefore, it is desirable that the time from when a circuit substrate is deflected to when the vibration decays and disappears is as short as possible.
If conventional ceramic substrate as described in Patent Document 1 have smaller thickness and larger size, deflection or the like may relatively occur due to residual stress which is generated at the time of sintering. If the ceramic substrate described in patent Document 1 is used as a substrate for holding sensors like the circuit substrate, there is a possibility that the decay time of vibration due to deflection may be increased.